Carbon or low-alloy steel with improved machinability and process of manufacture of that steel

ABSTRACT

A carbon or low-alloy steel is provided with improved machinability. The carbon content of the steel is less than 1.5% and the sum of the alloy elements contained therein is less than 9%. The steel contains oxide and sulfide inclusions, and the chemical composition of the steel includes 0.1%≦Mn, 0.01%≦Al≦0.05%, 0.025%≦S≦0.3%, 0.002%≦Ca, O≦0.0015% by weight; the “KO oxides” cleanness index is less than 30, all the oxides are lime aluminates, and the average calcium content of type 2, type 3, type 4 and type 5 inclusions is less than 30%. A process of manufacturing such steel and an oxide-coated wire for its manufacture are also provided.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention concerns a fine-grained carbon or low-alloy steelthat contains oxides and sulfides and is treated with calcium. The steelhas improved machinability and is capable of being continuously cast.

[0003] 2. Discussion of the Background

[0004] Carbon or low-alloy structural steels with improved machinabilityare well known. In these steels good machinability is obtained, notably,by the presence of relatively plastic sulflur-base inclusions and oxideinclusions having not too high a melting point, and by the absence ofhard abrasive inclusions. The sulfur-base inclusions facilitatemachining by favoring fragmentation of the machining cuttings, which isvery desirable for low-speed machining operations such as boring. Theoxide inclusions associated with calcium-rich manganese sulfides make itpossible to carry out high-speed machinings by forming, on the surfaceof the tool which makes contact with the cutting, a layer ensuring botha lubricating function and a diffusion-barrier function which retardstool wear and makes it possible to obtain a good surface state. Theabsence of hard abrasive inclusions avoids tool wear caused by frictionof such inclusions on the surface of the tool which makes contact withthe cutting.

[0005] The sulfur-base inclusions consist of manganese sulfide, in whichthe sulfur can be partially substituted by selenium or tellurium, andthe manganese can be partially substituted by calcium. Selenium,tellurium and calcium have the advantage of diminishing thehigh-temperature plasticity of manganese sulfides, which prevents thesulfur-base inclusions from becoming too elongated duringhigh-temperature, plastic deformation shaping operations and from overlydamaging the ductility of the metal. However, the calcium content of thesulfide inclusions should not be too high, because the mixed manganeseand calcium sulfide inclusions may then become hard and abrasive. Moreprecisely, the calcium content of the sulfide inclusions should be lessthan 75% and preferably less than 40%.

[0006] The oxide inclusions with not too high of a melting point are,notably, inclusions consisting of a lime aluminosilicate of suitablecomposition, in sufficient quantity and well dispersed. The oxideinclusions should not consist of pure alumina, an oxide that is, infact, extremely hard and is found in steel in the form of very abrasivemasses or alignments. In general, structural steels should contain alittle aluminum to control the size of the grain and, therefore, theycannot contain pure silica inclusions, which moreover are undesirable.

[0007] The above-described steels, which are obtained by deoxidationwith aluminum, resulfuration and injection with calcium or SICAL(SilicoCalciumAluminium), have a chemical composition characterizedspecifically by a sulfur content ranging between 0.025% and 0.3%, amanganese content exceeding 0.1%, an aluminum content ranging between0.1% and 0.05%, an oxygen content exceeding 0.002% and a calcium contentexceeding 0.002%. The rest of the composition is that of a standardcarbon or low-alloy structural steel, that is, it includes of iron,carbon, alloy elements, nitrogen and impurities resulting frommanufacture. Those steels can further contain supplementary additionelements, such as, for example, selenium, tellurium, bismuth or lead.

[0008] Beside the sulfur, manganese, aluminum, oxygen and calciumcontents, steels with improved machinability are characterized by thenature and quantity of the inclusions. The nature of the inclusionscorresponds to what has been defined above. The quantities are in directrelationship: sulfides with the sulfur content (and possibly selenium ortellurium content); and oxides with the oxygen content.

[0009] Even though they have a very good capacity for machining at bothlow speed and high speed, the above-identified steels inhere at leasttwo disadvantages. First, they require a high oxygen content andconsequently, a large quantity of oxide inclusions, which areunfavorable to fatigue strength. Second, they cannot be continuouslycast. In fact, the injection of SICAL or calcium leads to the formationof oxides or sulfides which clog the casting nozzles. Furthermore, whenthe oxygen content is insufficient, the injection of calcium leads tothe formation of calcium-rich sulfides, which are very hard andabrasive.

SUMMARY OF THE INVENTION

[0010] The purpose of the present invention is to solve theabove-described problems by offering a carbon or low-alloy steel withimproved machinability, treated with calcium, having a reduced oxygencontent and capable of being continuously cast.

[0011] Accordingly, the first embodiment of the present inventionrelates to a carbon or low-alloy structural steel with improvedmachinability, with a carbon content of less than 1.5% and the sum ofwhose alloy elements is less than 9%, containing oxide and sulfideinclusions and having a chemical composition by weight of:

0.1%≦Mn

0.01%≦Al≦0.05%

0.025%≦S≦0.3%

0.002%≦Ca

O≦0.0015%

[0012] the state of inclusion of that steel being characterized by a “KOoxides” cleanness index below 30, by the fact that all the oxideinclusions are lime aluminates and by an average calcium content of type2, type 3, type 4 and type 5 inclusions below 30%, and preferably below25%.

[0013] The second embodiment of the present invention relates to aprocess for the manufacture of a steel metallurgical product, including:

[0014] deoxidizing a molten steel with aluminum;

[0015] degassing the molten steel under vacuum;

[0016] injecting a powder that contains oxides whose melting temperatureis below 1500° C. into the molten steel; and

[0017] injecting calcium into the molten steel.

[0018] The third embodiment of the present invention relates to a coatedwire that contains an oxide powder having 50% to 70% SiO₂, 5% to 25%Al₂O₃, 5% to 25% CaO, 0% to 10% Fe₂O₃ and FeO, and a remainder beingimpurities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Various other objects, features and attendant advantages of thepresent invention will be more fully appreciated as the same becomesbetter understood from the following detailed description, which is notintended to be limiting unless otherwise specified.

[0020] The steel preferably does not contain type 5 inclusions. The type3 and 4 inclusions also preferably represent less than 30% of theinclusions. Finally, it is preferable for the proportion of type 2inclusions to be greater than 20% and for the average calcium content ofthose inclusions to range between 15% and 25% calcium.

[0021] The steel can be manufactured in the form of a metallurgicalproduct of continuously cast slab, bloom, billet, bar or wire type.

[0022] Preferably, the coated wire includes a tube that is fusible inthe molten steel and a powder being of a mixture of oxides having amelting temperature below 1500° C., more preferably below 1400° C. andmost preferably below 1300° C.

[0023] The oxide powder for the coated wire preferably contains 50% to70% SiO₂, 5% to 25% Al₂O₃, 5% to 25% CaO, 0% to 10% Fe₂O₃ and FeO, therest being impurities.

[0024] The oxide powder for the process preferably contains 50% to 70%SiO₂, 5% to 25% Al₂O₃, 5% to 25% CaO, 0% to 10% Fe₂O₃ and FeO, the restbeing impurities, and its composition is chosen so that its meltingtemperature will be preferably below 1500° C., more preferably below1400° C., and most preferably below 1300° C.

[0025] Preferably, the carbon or low-alloy structural steel withimproved machinability of the present invention is intended mainly forthe fabrication of machine parts obtained either by machining a steelbloom cut in a rolled steel product, or by machining a blank obtained byforging. The fabrication of these machine parts generally entails a heattreatment carried out on the part, or on a steel metallurgical product,or on the forged blank. The heat treatment may be completed by a surfacetreatment. The chemical composition of the structural steels used formachine parts may be chosen in accordance with the mechanical propertiesdesired, the use envisaged for the parts, and the size of the parts. Therange of composition possible for the steels of the present invention isvery wide and is not strictly defined. Preferably, however, these steelsgenerally contain:

[0026] carbon, between 0% and 1.5%, and preferably between 0.1 and 1.0%;

[0027] possibly at least one alloy element chosen among silicon,manganese, nickel, chromium and molybdenum (preferably replaced bytungsten), in quantities of less than 2% for silicon, 5% for manganese,6% for nickel, 5% for chromium and 2% for molybdenum, the sum of thosecontents generally being less than 9%, and preferably less than 7%;

[0028] possibly at least one microalloy element chosen among titanium,niobium and vanadium, in quantities of less than 1%, and possibly alsoboron, in quantities generally less than 0.002%. Beside the preferred,principal elements which define structural steels, the chemicalcomposition contains:

[0029] in general, aluminum, preferably in quantities less than 0.1%,more preferably less than 0.05%;

[0030] possibly elements such as sulfur, tellurium, selenium, bismuth,lead or rare earths, intended to improve machinability; when they arepresent, those elements are generally in quantities of less than 0.3%,more preferably less than 0.15%;

[0031] nitrogen, impurities and residues resulting from manufacture,particularly oxygen; the nitrogen content is generally less than 0.03%,more preferably less than 0.025%. The rest is iron, the content of whichexceeds 90%, more preferably 92%.

[0032] Preferably, to manufacture the steel of the present invention, araw molten steel is made in the converter or in the electric furnace andis then poured into a ladle. The standard ladle metallurgy operationsare carried out in the ladle, intended notably to deoxidize the steel byaddition of alumina, and to adjust the chemical composition by additionof ferroalloys. The molten steel is then degassed by treatment undervacuum, for example, by RH passage. The steel thus obtained, which ispractically ready to be cast in order to obtain a solid semifinishedproduct, is most preferably characterized, aside from its temperatureand content of carbon and alloy or microalloy elements, by its oxygen,aluminum, sulfur and manganese contents.

[0033] The aluminum which, on the one hand, is in the form of aluminumdissolved in the steel and, on the other, in the form of aluminaparticles, is present in an amount ranging between 0.01% and 0.05%, morepreferably between 0.015-0.03%.

[0034] The oxygen is most preferably combined with the aluminum in theparticles of alumina. Its content is less than 0.0015%, and morepreferably less than 0.001%. The oxygen content is particularlyimportant, since it represents, in fact, the quantity of aluminaparticles present in the steel, and those particles have a verysignificant bearing on the properties of the steel.

[0035] The sulfur, dissolved in the molten steel in a quantity rangingbetween 0.025% and 0.3%, and more preferably between 0.03-0.2%, isintended to form in the solid steel sulfide inclusions of elementshaving a strong affinity for sulfur, such as manganese or calcium. Thoseinclusions are all the more numerous and, therefore, are effective inimproving the machinability of the steel as the sulfur content is high.However, those inclusions are detrimental to certain mechanicalproperties of steel, such as ductility, which leads to limiting thesulfur content to values that are not too high, i.e., above 0.3%.

[0036] The manganese has several effects on the properties of the steeland, in particular, of being combined with the sulfur, as well as withother elements of the same family, e.g., with selenium or tellurium. Itscontent should be sufficient, when associated with other elements havinga strong affinity for sulfur, so that all of the sulfur and elements ofthe same family are preferably present in the solid steel in the form ofsulfide-type inclusions. The minimum manganese content should, for thatpurpose, exceed 0.1%, more preferably 0.15%.

[0037] The molten steel having the characteristics just described is notentirely ready to be cast. In fact, in order to obtain inclusions havingthe desired properties, namely, the absence of pure alumina, thepresence of lime aluminates, the presence of mixed manganese and calciumsulfides containing less than 30%, and more preferably less than 25%calcium, the oxide inclusions preferably being associated with sulfides,and the sulfide inclusions preferably containing between 15% and 25%calcium, it is necessary to introduce calcium in the steel. Furthermore,the inventors discovered that, for calcium to be introduced undersatisfactory conditions and to give the desired result, notably asufficient quantity of oxide inclusions, the introduction of calcium hadto be preceded by the introduction of suitably chosen oxide particles inthe steel.

[0038] Before casting the steel, a mixture of silica, alumina, lime andiron oxides having a melting point below 1500° C., preferably below1400° C., and most preferably below 1300° C. is introduced in the moltensteel, and then calcium is introduced by any method known to the expert,e.g., by means of a coated wire.

[0039] The chemical composition of the silica, alumina, lime and ironoxide mixture is preferably:

[0040] SiO₂ from 50% to 70%, more preferably approx. 60%

[0041] Al₂O₃ from 5% to 25%, more preferably approx. 16%

[0042] CaO from 5% to 25%, more preferably approx. 15%

[0043] Fe₂O₃ and FeO: from 0% to 10%, more preferably approx. 6%.

[0044] This mixture can additionally contain impurities and ispreferably manufactured in the form of a powder mixture fritted at hightemperature and contained in a mild steel tube, so as to have the formof a coated wire.

[0045] The above mixture can advantageously be obtained with a mixtureof approximately 40% fused clinker-base aluminous hydraulic cement andapproximately 60% powdered quartz sand.

[0046] Preferably, once treated with calcium, the molten steel is cast,in order to be solidified in the form of semifinished products which areshaped by plastic deformation, e.g., by rolling or by forging, so as toobtain, as the case may be, either a steel metallurgical product or aforged blank. Casting can be done in an ingot mold, but it can also bedone—and that is one of the major advantages of the invention—bycontinuous casting of billets, blooms or slabs.

[0047] The solid steel thus obtained contains alloy or microalloyelements characteristic of the grade of steel considered, but it alsocontains sulfur, on the one hand, and calcium and oxygen, on the other,all elements present in the form of inclusions. Surprisingly, theinventors discovered that, in spite of the introduction of the oxidemixture described above, the oxygen content was unaltered and remainedbelow 0.0015%. Furthermore, the calcium content exceeds 0.002%, and morepreferably 0.02% (that element, which is not very soluble in steel, ispresent essentially in the inclusions, and its average content in thesteel rarely exceeds 0.1%). The chemical composition of the steel istherefore characterized by the following contents:

0.1%≦Mn

0.01%≦Al≦0.05%

0.025%≦S≦0.3%

0.002%≦Ca

O≦0.0015%

[0048] To characterize the state of inclusion of the steel, on the onehand, one determines a “KO oxides” index according to German standardSEP 1570-71 and, on the other, by means of an electron scanningmicroscope coupled with an image analyzer and a retrodiffused electronchemical analysis device (EDS), one determines by automatic counting theproportion of inclusions belonging to each of the following classes:

[0049] type 1: filiform sulfides,

[0050] type 2: globular sulfides,

[0051] type 3: sulfides containing one or more oxides,

[0052] type 4: sulfides and oxides paired,

[0053] type 5: oxides alone.

[0054] The average calcium content of the inclusions belonging to eachof the above classes is also determined, the calcium contentcorresponding to the proportion of manganese which is substituted bycalcium in the sulfides (value of x in the formula Mn_(1-X)Ca_(X)S). Thecount is made on a population of over 200 inclusions. The filiformsulfides are distinguished from the globular sulfides by their shapefactor defined by the ratio of length L to width 1. For type 1, L/1 ismore than 3, and for type 2, L/1 is less than 3.

[0055] The inclusion population of the steel obtained by the processaccording to the invention is such that:

[0056] the “KO oxides” index is less than 30,

[0057] all the oxide inclusions are lime aluminates,

[0058] the average calcium content of the type 2, type 3, type 4 andtype 5 inclusions is less than 30% and more preferably less than 25% andgenerally ranges between 15% and 25%,

[0059] the average calcium content of the type 5 inclusions is less than30%,

[0060] in general, over 75% of the oxide inclusions are associated withsulfide inclusions,

[0061] the average calcium content of the type 1 inclusions generallyranges between 1% and 2%.

[0062] It is to be noted that the compositions of the inclusions can bedetermined by electron nicroprobe or macroprobe analysis. Thesecharacteristics of the state of inclusion make it possible to obtaingood machinability on high-speed machining, for example, on turning.

[0063] It is to be noted that the expert can easily distinguish productsobtained by continuous casting from products obtained by ingot castingby making the standard metallographic examinations.

EXAMPLES

[0064] Having generally described this invention, a furtherunderstanding can be obtained by reference to certain specific exampleswhich are provided herein for purposes of illustration to only and arenot intended to be limiting unless otherwise specified.

[0065] All the examples concern structural steels of 42CrMo4 and39NiCrMo type according to European standard EN 10083.1 and a 27CrMo4steel intended for automotive applications, the nominal compositions(average composition sought for the principal elements) of which are in% by weight: C Si Mn Ni Cr Mo 42CrMo4 0.42 0.25 0.7 — 1 0.25 27CrMo40.27 0.25 0.7 — 1 0.25 39NiCrMo3 0.39 0.25 0.6 0.75 0.8 0.2 

[0066] Those steels were manufactured by aiming at a sulfur content ofapproximately 0.03% and they were deoxidized with aluminum. Thefollowing castings were made:

[0067] AC, according to the invention (degassed, injection of anoxide-coated wire, injection of calcium), continuous castings,

[0068] BC, by way of comparison, degassed, with injection of calcium(without injection of oxide-coated wire), continuous castings,

[0069] AL, by way of comparison, not degassed, with injection of calcium(without injection of oxide-coated wire), ingot castings,

[0070] CL, by way of comparison, degassed, without injection of calciumor of oxide-coated wire, continuous castings.

[0071] Castings AC, BC and AL correspond to steels with improvedmachinability. The CL castings correspond to a standard steel, whichdoes not have improved machinability.

[0072] All the castings, except for those which could not besatisfactorily poured in continuous casting, were hot-rolled to formdisks approximately 100 mm in diameter, which were tempered and annealedto give them their properties of use. The aluminum, oxygen, sulfur andcalcium contents were then determined, the state of inclusion wascharacterized and turning machinability was evaluated by measurement ofthe V20B0.3 index (according to standard ISO 3685), giving index 100 tothe CL castings for machinability.

[0073] The following results were obtained:

[0074] conditions of casting and chemical analysis (in % by weight):Continuous Casting Ingot Al Ca S O AC good — 0.014 0.0025 0.031 0.0007AL — good 0.011 0.0024 0.026 0.0022 BC poor — 0.015 0.0020 0.027 0.0006CL good — 0.013 — 0.029 0.0012

[0075] state of inclusion and machinability: Nature of KO type 1 type 2type 3 type 4 type 5 V20 Oxides oxides nb % Ca % nb % Ca % nb % Ca % nb% Ca % nb % Ca B0.3 AC lime 10 30 1.3 45 20 11 18 14 25 0 0 151aluminates BC lime 18 42 2.3 38 31 4 30 16 46 0 0 113 aluminates AL lime60  9 0.3 20 13 11 22 60 33 0 0 130 aluminates CL aluminas 10.5 54 0.2 0  0  6 0.3  8 0.2 32  0.1 100

[0076] Those results clearly show that:

[0077] the steel according to the invention can be continuously cast,while a similar steel, degassed and treated with calcium withoutinjection of oxide-coated wire (BC castings) poses problems for thosecastings; in fact, the continuous casting was hampered by clogging ofnozzles;

[0078] the steel according to the invention (AC castings) shows bettermachinability than the steels with improved machinability not degassedaccording to the prior art (AL castings), in spite of a very appreciablylower lime aluminate content and a lower “KO oxides” index.

[0079] Having now fully described the invention, it will be apparent toone of ordinary skill in the art that many changes and modifications canbe made thereto without departing from the spirit or scope of theinvention as set forth herein.

[0080] This application is based on French Patent Application No. 9708449, filed Jul. 4, 1997, the entire contents of which are herebyincorporated by reference.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A carbon or low-alloy steel, comprising: acarbon content of less than 1.5% by weight, alloy elements in a combinedamount of less than 9% by weight, oxide and sulfide inclusions, whereinthe chemical composition of the steel comprises, by weight: 0.1%≦Mn0.01%≦Al≦0.05% 0.025%≦S≦0.3% 0.002%≦Ca O≦0.0015% wherein a “KO oxide”cleanness index is below 30, the oxide inclusions are lime aluminates,and an average calcium content of an inclusion selected from the groupconsisting of type 2, type 3, type 4 and type 5 inclusions, and mixturesthereof is less than 30%.
 2. The steel according to claim 1 , wherein itdoes not contain type 5 inclusions.
 3. The steel according to claim 1 ,wherein the number of type 3 and type 4 inclusions constitute less than30% of the total number of inclusions.
 4. The steel according to claim 1, wherein the number of type 2 inclusions is greater than 30% of thetotal number of inclusions and wherein the average calcium content ofthe type 2 inclusions is between 15% and 25%.
 5. A steel metallurgicalproduct selected from the group consisting of slab, bloom, billet, barand wire, comprising the steel according to claim 1 .
 6. The steelmetallurgical product according to claim 5 , wherein said product ismade by continuous casting.
 7. A process for the manufacture of thecarbon or low-alloy steel according to claim 1 , comprising: deoxidizinga molten steel with aluminum; degassing the molten steel under vacuum;injecting a powder comprising oxides whose melting temperature is below1500° C. into the molten steel; and injecting calcium into the moltensteel.
 8. The process according to claim 7 , further comprising castingthe molten steel.
 9. A process for the manufacture of carbon orlow-alloy steel, comprising: deoxidizing a molten steel with aluminum;degassing the molten steel under vacuum; injecting a powder comprisingoxides whose melting temperature is below 1500° C. into the moltensteel; and injecting calcium into the molten steel.
 10. The processaccording to claim 9 , wherein the oxide powder comprises 50% to 70%SiO₂, 5% to 25% Al₂O₃, 5% to 25% CaO, 0% to 10% Fe₂O₃ and FeO, and aremainder being impurities.
 11. The process according to claim 9 ,wherein the oxide powder has a melting temperature below 1400° C. 12.The process according to claim 9 , further comprising continuouslycasting the molten steel.
 13. The process according to claim 9 , whereinthe oxide powder is comprised within a tube that is fuisible in themolten steel.
 14. A coated wire, comprising an oxide powder comprising50% to 70% SiO₂, 5% to 25% Al₂O₃, 5% to 25% CaO, 0% to 10% Fe₂O₃ andFeO, and a remainder being impurities.
 15. The coated wire according toclaim 14 , wherein said oxide powder is comprised within a tube that isfuisible in molten steel.
 16. The coated wire according to claim 14 ,wherein said oxide powder has a melting temperature below 1500° C. 17.The coated wire according to claim 16 , wherein the melting temperatureis below 1400° C.
 18. The coated wire according to claim 17 , whereinthe melting temperature is below 1300° C.
 19. A process for themanufacture of carbon or low-alloy steel, comprising injecting thecoated wire according to claim 14 into a molten steel.